In addition to nematic and cholesteric liquid crystals, optically active tilted smectic (ferroelectric) liquid crystals have also been used recently in commercial display devices.
Clark and Lagerwall have been able to show that the use of ferroelectric liquid crystals (FLCs) in very thin cells results in optoelectrical switching or display elements which have response times faster by a factor of 1000 compared with conventional TN ("twisted nematic") cells (cf., for example, EP-A 0 032 362). On the basis of this and other favorable properties, for example the possibility of bistable switching and the virtually viewing angle-independent contrast, FLCs are fundamentally highly suitable for areas of application such as computer displays.
For the use of FLCs in electro-optical or fully optical components, either compounds are required which form tilted or orthogonal smectic phases and are themselves optically active, or ferroelectric smectic phases can be induced by doping compounds which, although forming such smectic phases, are not themselves optically active, with optically active compounds. The desired phase should be stable over the broadest possible temperature range.
In order to achieve good contrast in electro-optical components, a uniform planar alignment of the liquid crystals is necessary. Good alignment in the S.sub.A and S*.sub.C phase can be achieved, for example, if the phase sequence of the liquid-crystal mixture is, with decreasing temperature: EQU isotropic.fwdarw.N*.fwdarw.S.sub.A .fwdarw.S*.sub.C
The prerequisite is that the pitch of the helix in the N* phase is very large (greater than 10 .mu.m) or even better is fully compensated (see, for example, T. Matsumoto et al., pp. 468-470, Proc. of the 6th Int. Display Research Conf., Japan Display, Sep. 30 -Oct. 2, 1986, Tokyo, Japan; M. Murakami et al., ibid. pp. 344-347). This is achieved by adding one or more optically active dopes which induce a right-hand helix to the chiral liquid-crystal mixture which has, for example, a left-hand helix in the N* phase, in such amounts that the helix is compensated.
A further prerequisite for the use of the SSFLCD effect (surface-stabilized ferroelectric liquid-crystal display) of Clark and Lagerwall for uniform planar alignment is that the pitch in the smectic C* phase is significantly greater than the thickness of the display element (Mol. Cryst. Liq. Cryst. 94 (1983), 213-134 and 114 (1984), 151-187). As in the case of the cholesteric pitch, this is achieved by using dopes having the opposite rotation of the helix.
The optical response time .gamma. [.mu.s] of ferroelectric liquid-crystal systems, which should be as short as possible, depends on the rotational viscosity of the system y[mPas], the spontaneous polarization P.sub.s [nC/cm.sup.2 ] and the electric field strength E[V/m], in accordance with the equation ##EQU1## Since the field strength E is determined by the electrode separation in the electro-optical component and by the applied voltage, the ferroelectric display medium must have low viscosity and a high spontaneous polarization to achieve a short response time.
Finally, in addition to thermal, chemical and photochemical stability, a small optical anisotropy .DELTA.n, preferably .apprxeq.0.13, and a low positive or preferably negative dielectric anisotropy .DELTA..epsilon. are required (see, for example, S. T. Lagerwall et al., "Ferroelectric Liquid Crystals for Displays", SID Symposium, Oct. Meeting 1985, San Diego, Calif., USA).
The totality of these requirements can only be achieved by means of mixtures comprising a plurality of components. The base (or matrix) used preferably comprises compounds which if possible themselves already have the desired phase sequence I.fwdarw.N.fwdarw.S.sub.A .fwdarw.S.sub.C. Further components of the mixture are frequently added in order to reduce the melting point and to broaden the S.sub.C and usually also the N phase, to induce optical activity, for pitch compensation and to match the optical and dielectric anisotropy; further, the rotational viscosity, for example, should if possible not be increased.
Ferroelectric liquid-crystal displays can also be operated by utilizing the DHF (distorted helix formation) effect or the PSFLCD effect (pitch-stabilized ferroelectric liquid-crystal display, also known as SBF=short pitch bistable ferroelectric effect). The DHF effect has been described by B. I. Ostrovski in Advances in Liquid Crystal Research and Applications, Oxford/Budapest, 1980, 469 ff.; the PSFLCD effect is described in DE-A 39 20 625 and EP-A 0 405 346. In contrast to the SSFLCD effect, utilization of these effects requires a liquid-crystalline material having a short S.sub.C pitch.
Since the development of ferroelectric liquid-crystal mixtures in particular can in no way be regarded as complete, the manufacturers of displays are interested in a very wide variety of components for mixtures. Another reason for this is that only the interaction of the liquid-crystalline mixtures with the individual components of the display device or of the cells (for example the alignment layer) allows conclusions to be drawn on the quality of the liquid-crystalline mixtures too.
The object of the present invention was therefore to provide novel compounds which are suitable in liquid-crystalline mixtures for improving the property profile of these mixtures.
JP-A-62/195,355 and EP-A-0 546 338 describe indane derivatives and their use in liquid-crystal mixtures.